Add support for UIPickerViews in Subliminal

UIPickerViews are under a different Window other than keyWindow, so I had to find the other one.  Otherwise it is pretty straightforward.  I'm leaning on the UIA methods a bit more heavily for interacting with the element, but open to suggestions.

Added one fairly comprehensive test case.
Can you document these? Its super nice for library users since the docs show up in autocomplete info and the sidebar + Travis fails without it https://travis-ci.org/inkling/Subliminal/jobs/27543975